The invention relates to medical devices, such as, for example, stents and stent-grafts, and methods of making the devices.
The body includes various passageways such as arteries, other blood vessels, and other body lumens. These passageways sometimes become occluded or weakened. For example, the passageways can be occluded by a tumor, restricted by plaque, or weakened by an aneurysm. When this occurs, the passageway can be reopened or reinforced, or even replaced, with a medical endoprosthesis. An endoprosthesis is typically a tubular member that is placed in a lumen in the body. Examples of endoprosthesis include stents and covered stents, sometimes called xe2x80x9cstent-graftsxe2x80x9d.
Endoprostheses can be delivered inside the body by a catheter that supports the endoprosthesis in a compacted or reduced-size form as the endoprosthesis is transported to a desired site. Upon reaching the site, the endoprosthesis is expanded, for example, so that it can contact the walls of the lumen.
The expansion mechanism may include forcing the endoprosthesis to expand radially. For example, the expansion mechanism can include the catheter carrying a balloon, which carries a balloon-expandable endoprosthesis. The balloon can be inflated to deform and to fix the expanded endoprosthesis at a predetermined position in contact with the lumen wall. The balloon can then be deflated, and the catheter withdrawn.
In another delivery technique, the endoprosthesis is formed of an elastic material that can be reversibly compacted and expanded, e.g., elastically or through a material phase transition. During introduction into the body, the endoprosthesis is restrained in a compacted condition. Upon reaching the desired implantation site, the restraint is removed, for example, by retracting a restraining device such as an outer sheath, enabling the endoprosthesis to self-expand by its own internal elastic restoring force.
To support a passageway open, endoprostheses are sometimes made of relatively strong materials, such as stainless steel or Nitinol (a nickel-titanium alloy), formed into struts or wires. These materials, however, can be relatively radiolucent. That is, the materials may not be easily visible under X-ray fluoroscopy, which is a technique used to locate and to monitor the endoprostheses during and after delivery. To enhance their visibility (e.g., by increasing their radiopacity), the endoprostheses can be coated with a relatively radiopaque material, such as gold. Because the endoprostheses are typically kept in the body for a relatively long time, it is desirable that they have good biocompatibility.
The invention relates to methods of making medical devices, such as, for example, stents and stent-grafts, and methods of making the devices. More particularly, the invention features an endoprosthesis, such as a stent, having a layer that can enhance the biocompatibility of the endoprosthesis.
In one aspect, the invention features a stent including a member having a first portion, and a second portion disposed outwardly of the first portion. The second portion is more radiopaque than the first portion and has a first layer including a radiopaque material, and a second layer defining an outer surface of the member and including the radiopaque material and a second material.
Embodiments may include one or more of the following features. The second layer includes an alloy of the radiopaque material and the second material. The radiopaque material is selected from the group consisting of gold, platinum, palladium, and tantalum. The second material is selected from the group consisting of titanium, chromium, palladium, niobium, and silicon. The first portion includes a material selected from the group consisting of stainless steel and nickel-titanium alloy.
The first portion can be the innermost portion of the member, and/or contact the second portion.
The stent can further include a third portion between the first portion and the second portion, a polymeric layer on the member, and/or a drug-releasing layer on the member.
In another aspect, the invention features a stent including a member having a first portion having a first layer including a radiopaque material, and a second layer defining an outer surface of the member and including the radiopaque material and a second material.
In another aspect, the invention features a stent including a member having a first portion, and a second portion disposed outwardly of the first portion. The second portion is more radiopaque than the first layer and includes a first layer having a radiopaque material, and a second layer including the radiopaque material and defining an outer surface of the member, the second layer having a lower oxidation potential than an oxidation potential of the first layer.
Embodiments may include one or more of the following features. The radiopaque material is selected from the group consisting of gold, platinum, palladium, and tantalum. The second layer includes an alloy of the radiopaque material and a second material. The second material is selected from the group consisting of titanium, niobium, palladium, chromium, and silicon.
The first portion can include a material selected from the group consisting of stainless steel and a nickel-titanium alloy. The first portion can be the innermost portion of the member. The first portion can contact the second portion.
The first and second portions can have different compositions.
The stent can further include a polymeric layer on the member and/or a drug-releasing layer on the member.
In another aspect, the invention features a stent having a member having a first portion including a first layer comprising a radiopaque material, and a second layer comprising the radiopaque material and defining an outer surface of the member. The second layer has a lower oxidation potential than an oxidation potential of the first layer.
In another aspect, the invention features a stent having a member including a first portion having a concentration gradient of a radiopaque material, the first portion defining an outer surface of the member.
Embodiments may include one or more of the following features. The concentration of the radiopaque material increases as a function of distance from the outer surface. The concentration gradient varies substantially linearly along a thickness of the first portion. The radiopaque material is selected from a group consisting of gold, platinum, palladium, and tantalum. The first portion is formed of an alloy including the radiopaque material and a second material. The member further includes a second portion disposed inwardly of the first portion, the second portion being more radiolucent than the first portion.
In another aspect, the invention features a method of making a stent including a member. The method includes forming an outer layer on the member having a radiopaque material and a second material, and oxidizing a portion of the outer layer.
Embodiments may include one or more of the following features. Oxidizing the portion includes forming an oxide or a nitride from the outer layer. The method further includes forming a radiopaque layer having the radiopaque material. The outer layer is formed with a compositional gradient.
The outer layer is formed by a process selected from the group consisting of physical vapor deposition, chemical vapor deposition, and electrodeposition.
Oxidizing the portion of the outer layer can be performed by electropolishing, by heating the outer layer in an oxidizing environment, and/or by ion implanting oxygen in the outer layer and heating the outer layer.
The method can further include forming a polymeric layer on the outer layer, and/or forming a drug-releasing layer on the outer layer.
Other aspects, features and advantages of the invention will be apparent from the description of the preferred embodiments thereof and from the claims.